The invention relates generally to ultrasonic devices, and more specifically to laser ultrasonic devices.
Laser-ultrasonic devices use lasers to generate and detect ultrasonic waves within solid samples. Techniques based on such laser ultrasonic devices are non-contact techniques, and are useful to measure a variety of material properties, such as the presence of defects. The basic components of a laser ultrasonic device are an ultrasonic generation system and an ultrasonic detection system.
The ultrasonic generation system usually consists of a high power generation laser capable of producing short pulses (typically of duration lying between femtoseconds to tens of nanoseconds) of laser energy. Many currently used generation lasers are solid state Q-Switched Nd:YAG and gas lasers (carbon dioxide or Excimers).
One known laser based ultrasonic generation technique utilizes the principle of thermoelastic expansion. Specifically, ultrasonic waves are generated on the surface or within the bulk of a sample by a sudden thermal expansion due to a sudden heating of a small surface of the sample material by the pulse of laser energy from the generation laser. Another known laser based ultrasonic generation technique is based on laser ablation of a sample.
Known ultrasonic detection systems usually include a detection laser, used to provide an interrogating beam of laser energy, and an interferometry based detection scheme. Typically, a detection laser provides continuous or long pulses (typically of a duration up to tens of microseconds) of laser energy and with long coherence length. The interrogating beam is scattered by the sample surface, wherein the sample surface is perturbed by an arrival of ultrasonic waves. The interferometry based detection scheme is then used to detect this perturbation.
A laser ultrasonic system that has a high signal-to-noise ratio, is compact enough to enable detection of flaws or defects in otherwise difficult to access locations, and is economical to construct and maintain is highly desirable.